Production of oil and gas from off-shore fields has created many unique engineering challenges. One of these challenges is dealing with effects of currents on long and unsupported pipe segments. These currents cause vortexes to shed from the sides of the pipes, inducing vibration. These vibrations can eventually lead to fatigue failures of the pipes.
For short segment of pipe, the pipe can be made strong enough to resist significant movement by the forces created by the shedding of vortexes. Alternatively, the pipe could be braced to prevent such movement or to change the frequencies at which the pipe would be excited by vortex shedding.
When unsupported segments of pipes must be very long, strengthening or bracing becomes impractical. Deep water production risers, drilling risers, platform export risers, and tendons for tension leg platforms are examples of pipes that are difficult or impossible to brace sufficiently to prevent some vibration due to vortex shedding. Pipelines traversing valleys on the ocean floor can also be unsupported for extended lengths.
Shrouds, strakes and fairings have been suggested for application to subsea pipes to reduce vortex induced vibrations. Strakes and shrouds can be made to be effective regardless of the relative direction of the current. But shrouds and strakes are generally less effective than fairings. Fairings are generally very effective in reducing vibrations due to vortex shedding, and also reduce drag forces on the pipe. But fairings have been thought to be effective only if the relative direction of the current is parallel to the axis of the fairing. U.S. Pat. Nos. 4,398,487 and 4,474,129 disclose fairings for use with subsea pipes and risers, and both of these patents disclose means to permit the fairing to rotate around the pipe or riser. The need to provide a rotatable fairing results in a very expensive apparatus. Further, the subsea environment in which the fairings are intended to operate will likely eventually lead to failure of the fairing to rotate. These references prefer fairings having a length to width ration of two or greater, so failure of the fairing to rotate would result in excessive drag forces being exerted on the pipe when the current is not aligned with the fairing. These rotatable fairings have therefore only been applied to drilling risers, not risers that would remain in service for extended time periods.
It is therefore an object of the present invention to provide a fairing that is effective to reduce vibration of a tubular caused by vortex shedding. It is a further object to provide such a fairing wherein the fairing can be provided in a fixed position on a tubular, and not cause significant increases in drag forces or vortex induced vibration when currents are not parallel to the axis of the fairing. In another aspect, it is an object of the present invention to provide a method to control vortex shedding induced vibrations utilizing such a fairing.